It really depends on what you are going to receive the temperature data on.
If you are using an Arduino as the receiver , its easy to build your own temperature sensor that looks like an Oregon Sensor as much of the OSv3 protocol has been reverse engineered.
However if you want to use an Oregon Console as the receiver then it gets a lot harder as not all the data fields
in the OSv3 protocol are understood and Oregon uses a modified form of manchester coding thats not easy to generate.